Communication pubs.acs.org/IC
High-Temperature Spin Crossover in a Mononuclear Six-Coordinate Cobalt(II) Complex † ́ Joanna Palion-Gazda,† Anna Switlicka-Olszewska, Barbara Machura,*,† Thais Grancha,‡ Emilio Pardo,‡ ‡ ,‡ Francesc Lloret, and Miguel Julve* †
Department of Crystallography, Institute of Chemistry, University of Silesia, 9th Szkolna Street, 40006 Katowice, Poland Departament de Química Inorgànica, Instituto de Ciencia Molecular (ICMol), Universitat de València, 46980 Paterna, València, Spain
‡
S Supporting Information *
Herein we present a preliminary magnetostructural study on the cobalt(II) complex of formula [Co(tppz)2](tcm)2 (1; tcm = tricyanomethanide anion), which exhibits a thermally induced spin transition with a change of the electron configuration t2g6eg1 (2Eg, LS) ⇔ t2g5eg2 (4T1g, HS), the LS phase being achieved at T ≤ 200 K. The crystal structure of 1 has been solved at 293 K (Table S1 in the Supporting Information, SI). It consists of discrete [Co(tppz)2]2+ cations and tcm− counteranions (Figure 1).
ABSTRACT: The six-coordinate cobalt(II) complex of formula [Co(tppz)2](tcm)2 exhibits a thermally induced spin-crossover behavior from a high spin (S = 3/2) at higher temperatures to a low spin (S = 1/2) at lower temperatures, with the low-spin phase being achieved at T ≤ 200 K.
A
lthough the preparation of the 2,3,5,6-tetrakis(2-pyridyl)pyrazine ligand (hereafter denoted as tppz) was first reported in 1959 by Goodwin and Lions,1 the first structural report of a tppz-containing metal complex appeared only 20 years ago.2 The apparent lack of interest in the investigation of the coordination chemistry of such a prototype of a ligand with two 2,2′:6′,2″-terpyridine (terpy) domains was based the assumption that tppz would coordinate to only one metal ion because the four pendant 2-pyridyl rings could not be coplanar with the central pyrazine ring. In spite of confirmation of this lack of coplanarity,3 several studies have revealed the great versatility of this ligand in complex formation, adopting a variety of terminal (bidentate α or γ and tridentate) and bridging [bis(bidentate) α or γ, bis(bidentate) α and γ, and bis(tridentate)] coordination modes.4 Applications of its metal complexes as sensors,5 molecular wires,6 biochemical and DNA binding properties,7 supramolecular films,8 precursors to synthesize carbonsupported catalysts (Fe−Nx/C),9 and builders for the rational design of extended structures containing [Mo 5 O 15 )(HOPO3)2]4−,10a [Mo5O15(O3PR)2]4−,10b,c and Mo8O264−10d−f clusters have been reported. Another property of tppz is its ability to mediate relatively large magnetic interactions between paramagnetic centers separated by more than 6.4 Å in the tppz-bridged metal complexes.4b,11−14 Interestingly, the [Fe(tppz)2]2+ entity is a low-spin (LS) species,4c,12 with the tridentate tppz molecule behaving as a strong-field ligand versus iron(II) in analogy with the parent terpy ligand.15 Keeping in mind the greater mean spin-pairing energy for cobalt(II) relative to that for iron(II) for comparable six-coordinate complexes,16 the possibility of the [Co(tppz)2]2+ species to undergo a thermally induced spin transition deserves to be explored. It is worthe noting that the majority of spincrossover (SCO) compounds characterized so far deal with iron(II) and iron(III) compounds, whereas those with cobalt(II) have been investigated to a lesser extent.15b © 2014 American Chemical Society
Figure 1. Perspective view of 1 (50% probability ellipsoids).
Each mer-tridentate tppz ligand coordinates to the cobalt(II) ion equatorially via two pyridyl donors and axially via the pyrazine, completing the coordination sphere. The value of the N(pyrazine)−Co−N(pyrazine) angle [179.56(8)°] is quasi-linear, whereas those of the remaining trans N−Co−N angles [156.80(7)° and 163.15(8)° for N(9)−Co(1)−N(11) and N(3)−Co(1)−N(5), respectively] reflect the steric constraints imposed by the rigidity of the tridentate tppz ligand. The dihedral angle between the least-squares planes defined by the coordinated pyridine and pyrazine rings of the two ligands is 86.35°, and the distortion parameter Σ, defined as the sum of the deviation from 90° of the 12 cis angles of the CoN6 octahedron, equals 81.70°. The axial Co−Npyrazine bond lengths [1.865(2) and 1.936(2) Å] are shorter than the Co−Npyridyl ones [1.977(2)− Received: May 22, 2014 Published: September 8, 2014 10009
dx.doi.org/10.1021/ic501195y | Inorg. Chem. 2014, 53, 10009−10011
Inorganic Chemistry
Communication
2.160(2) Å; Table S2 in the SI), forming a compressed octahedral surrounding, as expected for a LS six-coordinate cobalt(II) ion significantly distorted by Jahn−Teller arising from the single eg electron.17 The structural parameters of 1 are consistent with a thermal-averaged mixture of HS and LS states for cobalt(II) SCO complexes (see Table S4 in the SI). The whole ligands are appreciably twisted: the dihedral angles between the N(1)/N(2) pyrazine ring and each of its pyridyl substituents are 43.73(9)°, 45.94(10)°, 14.13(10)°, and 17.39(10)° arranged in order of rings containing N(4), N(6), N(5), and N(3) [to be compared with 23.52(11)°, 24.74(11)°, 31.31(11)°, and 35.00(10)° for the N(11), N(9), N(10), and N(12) rings in the tppz ligand containing the N(7)/N(8) pyrazine group]. The pyrazine rings from the two tppz ligands are significantly puckered, with maximum atomic deviations from a best mean plane being 0.0810(16) and 0.1313(17) Å at the C(3) and C(26) atoms, respectively. Low-energy conformations of free tppz molecules are predicted to have each pyridyl ring twisted from coplanarity with the pyrazine group roughly 50°.4d π−π-type interactions involving the pyridyl rings of the tppz ligands, which connect the [Co(tppz)2]2+ cations into a chain with cobalt(II) ions separated by 8.940 Å, contribute to stabilization of the structure {values of 3.8971(15) and 3.8366(16) Å for the Cg1[N(5)−C(20)−C(24)]···Cg2i[N(3)−C(8)−C(12)] and Cg3[N(12)−C(39)−C(43)]···Cg4i[N(10)−C(34)−C(38)] centroids, respectively; i, −1 + x, y, z; Figure 2}. Additionally, the tcm− anions are linked to the
Figure 3. χMT versus T plot for 1: (○) experimental; () best-fit curve through a model derived elsewhere.19 The inset shows the M versus H plot for 1 at 2.0 K.
combination of spin−orbit coupling and first- and second-order Zeeman effects,19 leads to the following two sets of best-fit parameters: γ = −1.10(1), μE = 1.91 μB, ζ = 510 cm−1, E = 2015(10) cm−1, and C = 0.075(1) [or γ = −1.49(1), μE = 1.86 μB, ζ = 450 cm−1, E = 2070(10) cm−1, and C = 0.0105(3)]. γ is a term that lies between −1.5 (weak-field ligand) and −1 (strong-field ligand), μE is the effective magnetic moment for the 2E state, ζ is the one-electron spin−orbit coupling constant, E is the energy gap between the 4T and 2E states, and C is the ratio of the vibrational partition functions in the HS and LS states. The calculated curves through both sets of values match very well the experimental data in the whole temperature range explored, and they compare well with those reported for other CoIIN6 SCO systems.20 X-band electron paramagnetic resonance (EPR) measurements on a polycrystalline sample of 1 at low temperatures consists of a quasi-isotropic feature centered at 3220 G (Figure S2 in the SI; T = 25 K). This signal for a frozen dichloromethane solution of 1 at 4.0 K shows a hyperfine splitting of A = 70 G in the form of eight lines due to coupling to the I = 7/2 nucleus of cobalt, with the g value being 2.12 and a frozen dichloromethane solution (Figure 4).
Figure 2. View of a fragment of the supramolecular chain in 1 arising from weak π−π-type interactions.
complex cation via very weak C−H···N-type interactions (Table S3 in the SI), leading to a three-dimensional supramolecular network. These intramolecular interactions seem to generate additional chemical pressure to stabilize the LS state of cobalt(II) in 1. This is reflected by the low value of the C parameter (see below). A crucial role of lattice interactions in driving the SCO behavior has been evidenced previously.15a,18 The χMT versus T plot for 1 [χM is the magnetic susceptibility per cobalt(II) ion] is shown in Figure 3. χMT is almost constant in the temperature range 1.9−185 K, taking a value of ca. 0.42 cm3 mol−1 K. This value is consistent with the S = 1/2 ground state that follows the Curie−Weiss law with g = 2.12. When the sample is warmed, χMT smoothly increases, attaining a value of 0.85 cm3 mol−1 K at 350 K. This behavior clearly indicates the occurrence of a very incomplete 1/2 (LS) ⇔ 3/2 (HS) conversion at 350 K. The magnetization (M) versus H plot for 1 at 2.0 K (see the inset of Figure 3) corresponds to what is expected for a spin doublet, with the value of M at 5 T (the maximum available field in our magnetometer) tending to a value somewhat above 1.0 μB Analysis of the magnetic data of 1 through a theoretical model that considers one doublet state (2E) and one quartet state (4T1) separated by an energy gap E, whose degeneracy is removed by a
Figure 4. X-band EPR spectrum of a frozen dichloromethane solution of 1 at 4.0 K.
In summary, the tppz molecule having two potential tridentate sites in trans position is a strong-field ligand in its cobalt(II) complexes, as shown by the present magnetostructural investigation on the mononuclear compound 1. Variabletemperature magnetic susceptibility measurements on a polycrystalline sample and EPR spectroscopy on both solid and frozen dichloromethane solutions of 1 revealed SCO behavior, in agreement with the previous results on the parent 10010
dx.doi.org/10.1021/ic501195y | Inorg. Chem. 2014, 53, 10009−10011
Inorganic Chemistry
Communication
[Co(terpy)2]2+, [Co(4-terpyridone)2]2+, and [Co(dpzca)2] [terpy = 2,2′:6′,2″-terpyridine, 4-terpyridone = 2,6-bis[2pyridyl-4(1H)-pyridone], and Hdpzca = bis(2pyrazinylcarbonyl)amide] mononuclear species.16c,21 The LS state in 1 is achieved at T = 185 K with S = 1/2, and the HS configuration is far from being completely populated at 350 K, with the value of χMT at this temperature being only 0.85 cm3 mol−1 K (to be compared with values of χMT in the range 2.5−3.0 cm3 mol−1 K for full population of the HS state in the parent terpy- and 4-terpyridone-containing complexes). The influence of the counterion on the spin transition in 1 as well as the use of the [Co(tppz)2]2+ unit as a metalloligand to build SCO heterometallic species will be the subject of further works.
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis, crystallographic data (Tables S1−S3), CCDC 999841, structural data for CoN6 SCO systems (Table S4), UV−vis electron spectrum (Figure S1), and X-band EPR spectrum on a solid sample (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Polish National Science Centre (Grant 2012/07/N/ST5/02213), the MICINN (Spain; Project CTQ2010-15364), and the Generalitat Valenciana (Spain; Projects PROMETEO/2009/108 and ISIC/2012/002). Thanks are due to the Universitat de València for a predoctoral grant (T.G.), the Doktoris Project co-financed by the European Social Fund for a scholarship (J.P.G.) and the MICINN Ramón y Cajal Program for a postdoctoral contract (E.P.).
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REFERENCES
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